Otherwise, you're hitting a known issue with this series. The optical drive SATA port is only SATA I (1.5 Gb/s) unlike the HD's SATA port which is SATA II (3.0 Gb/s). The problem is the Platform Controller Hub chip used in this system.

Thanks, I already tried the updater, doesn't run, because my system already has the current firmware. SSD speeds are decent, I know that disc performance on this port is not ideal but ca. 200-250 MB read write which I'm getting now is ok for me, if only I could fix the slow CPU...

The optical drive sensor is one of many sensors within the system SMC series uses. There is a different thermal sensor within the LCD unit. You may want to review the Apple service manual and or schematic for the system which shows all of the thermal sensors or you can download this app: TG Pro while I do recommend buying the full version, you'll find the trail version ID's all of the thermal sensors within your system quite nicely.

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